This invention relates to new and useful improvements in grids for vapor-liquid contact apparatus.
In the chemical engineering art, there are many instances where mass transfer reactions, energy transfer reactions, and chemical reactions, or combinations of these are carried out by bringing a vapor and a liquid into intimate contact with each other, usually within a vessel such as a refining tower. In continuous processes, liquid and vapor feed stock streams are continuously introduced to the vapor-liquid contact vessel, and vapor and liquid product streams are continuously withdrawn. The flow paths of the two streams through the vessel are in most instances of the countercurrent type, with the liquid being introduced at or near the top of the vessel, and withdrawn at the bottom, and with the vapor being introduced at or near the bottom of the vessel and withdrawn at the top. In some instances, concurrent flow, with both streams moving through the vessel in the same direction, is employed.
It is the practice to mount within the vapor-liquid contact vessel passive apparatus or structure intended to insure that the liquid and vapor achieve the desired degree of contact with each other so that the planned reaction occurs at the designed rate. The internal structure is passive in the sense that it is not power driven and has few or no moving parts. (Those parts that do move do so under the influence of the vapor or liquid moving through the vessel.) Various kinds of structures have been employed, including bubble trays, packed columns, and grids.
In these passive vapor-liquid contact devices, an important goal is to present a structural and surficial geometry which encourages the liquid moving through the device to form itself into films having, in the aggregate, a large area past which the vapor sweeps. But the design problem is not merely a matter of providing a large amount of surface area, because a number of other interrelated considerations must be taken into account.
From a process standpoint, it is important that the desired vapor-liquid contact reaction be carried as close to completion as possible. For example, in a crude oil vacuum tower, close fractionation and good separation are needed to produce gas oil streams that are free of undesirable residual elements, such as solids, Conradson carbon, and metals which are sometimes present in the feed stock, and to produce a "bottoms" stream which is low in gas oil content, so that desired product is not lost. Thus, both product quality (purity) and quantity (yield) hinge on effective and efficient performance of the vapor-liquid contact reaction.
From an operational viewpoint, the contact vessel and its internal apparatus must utilize the heat supplied to the unit efficiently, to minimize direct operating costs, whether the reaction is mass transfer, heat transfer, liqid vaporization, or vapor condensing duty. Furthermore, the reaction should be accomplished with a minimum pressure drop, since provision of the required pressure or vacuum is also an operating cost.
It is also desirable that the internal vapor-liquid contact apparatus be corrosion resistant, and resistant to fouling and coking, in order to lengthen the time between maintenance shutdowns. By the same token, the apparatus should be easily cleaned, repaired, removed and replaced through vessel man-ways to shorten the downtime during maintenance "turnarounds", since such time is lost production time, and an indirect operating cost.
Turning next to constructional or capital considerations, the vapor-liquid contact apparatus should be simple and economical to build. Some contact devices have been proposed or even built and used, which have been excellent from the process and operational standpoints, but which have been difficult to fabricate because of their complexity, and thus represent excessive capital cost.
An efficient vapor-liquid contact apparatus utilizes less tower or vessel space than an inefficient one doing the same work. Efficiency thus bears directly on capital costs, and the effect is accentuated, since the costs of both the apparatus and the enclosing tower are directly proportional to the amount of material required to produce them.
The matter of tower size is particularly important from a capital cost standpoint, because some cost elements of towers accelerate with increase in size. It is economically important to utilize as small and as short a vessel as practical, commensurate with its desired capacity and efficiency, because the cost of handling, shipping and erecting a vessel is directly proportional to its size. Furthermore, the necessary wall thickness of a vessel designed to operate at a designated internal pressure is directly proportional to the diameter of said vessel; hence, a further saving in weight for a given vessel can be effected if the vessel can be made smaller in diameter by reason of a functionally more effective vapor-liquid contact internal apparatus. It is also a recognized engineering principle that the shorter a vessel may be designed, the less metal will be required in the vessel per se and in its foundation base for wind load requirements. Likewise, it should be understood that the wall thickness and stiffener supports will not require as much material for vacuum service when a tower can be reduced in diameter.
In the vapor-liquid contact art, vessels or towers vary widely in size, ranging from pilot plant towers of only a few inches in diameter, to towers in excess of 40 ft. - 0 inches in diameter, depending on the desired capacity or throughput. If the requirements dictate a tower in excess of 10 or 12 feet, auxiliary supports are normally required to support the tower internals, including the vapor-liquid contact apparatus. Such auxiliary supports require additional tower height and cost, because room must be provided for them.
Normally, a vapor-liquid contacting apparatus utilizing packing material of either the bulk type or grid type, or any other type, requires, for a given zone, a means of introducing the liquid into said zone, a means for uniformly distributing the liquid into said zone, a means for creating intimate vapor-liquid contacting in said zone and a means, if required, to collect the liquid after it has passed through the upper sections of said zone.
It has been normal in the art to handle the last three functions (liquid distribution, vapor-liquid contact, and liquid collecting) in whole or in part by separate units that are separately supported. In doing this, valuable space of height in the tower is utilized to provide room for the supporting structures for these three functional sections, and in many cases additional height of tower is needed to provide volumetric space for stabilizing either the ascending vapor flow, or descending liquid flow, or both.
In some vapor-liquid contact towers it is necessary to remove part of the liquid product in side streams at one or more elevations along the height of the tower. In accordance with conventional practice, collector means are mounted in the tower at such point, and a separate set of vapor-liquid contact equipment, including liquid introducing means, liquid distributing means, the vapor-liquid contact means itself, and the next lower collector means, are all positioned below the side stream drawoff point, together with the necessary supporting structure for this equipment, and tower space for stabilizing flow of vapor and liquid. In effect, then, the tower is broken into a series of vertically stacked segments, or zones, with a substantially complete set of internal tower equipment being located between each side stream drawoff point and the next higher or lower drawoff point.
Thus, in a tower with several side streams, conventional practice tends to drive the capital cost up, because the amount of space in the tower required for the auxiliary functions of liquid distribution and collection, mechanical support, and vapor flow stabilization is multiplied by the number of side streams. Tower space devoted to these auxiliary functions is just as expensive as space devoted to vapor-liquid contact, but contributes only indirectly to the carrying out of vapor-liquid contact reactions.
The foregoing considerations may be summarized as follows: it is desirable that vapor-liquid contact apparatus produce good product quality and yield, at a good thermal efficiency and low pressure drop; that it be practical and simple to construct and maintain; and that its size and the tower size be minimized while throughput capacity is maximized.